This invention relates generally to tile field of filtered connectors, and more specifically to a filtered connector including both a ferrite cylinder and a capacitor to filter noise, including electromagnetic interference, generated in the system of which the filtered connector is a part.
In the field of communications there are networks, such as local area networks used with computers, which use coaxial cables to transmit signals. For safety reasons, as well as others, BNC connectors used in computer network applications are not allowed to be tied directly to the chassis of the system enclosure. For example, in a 10 base-2 type network (IEEE standard 802.3) only one point can have a connection to the ground. All other points on the network must remain floating.
FIG. 1 depicts a typical application of the so called 10 base-2 Ethernet connection in a local area network, as discussed above, wherein the shell 40 of a connector 10' is isolated from the chassis enclosure panel 100. Here the data bits are sent at the rate of 10 MHz. The circuit is biased by a dedicated DC/DC converter, which provides an isolated -9 v output. This voltage is used by the transceiver to process the transmitted and received data. Normally, the -9 v return (-9 v ret in FIG. 1) is tied to the shell of the BNC connector. However, the whole I/O area must be galvanically isolated from the rest of the system.
If the circuit shown in FIG. 1 processes high pulsed current or large swings of voltage it may produce interference couples to the BNC connector. This undesired noise could be returned to the source via the chassis enclosure panel, were the BNC shell 40 tied to it. However, the isolation requirement prevents that solution. Thus, the noise may leave the enclosure, flow through a connected cable and then return to the source.
Prior art filtered coaxial connectors have been able to limit the amount of spurious signals transmitted via the cable by using a capacitor (70 of FIG. 1) to shunt some of these signals to the chassis. U.S. Pat. No. 4,797,120 to Ulery, issued Jan. 10, 1989; U.S. Pat. No. 4,884,982 to Fleming et al., issued Dec. 5, 1989; U.S. Pat. No. 5,062,811 to Hackman, issued Nov. 5, 1991; U.S. Pat. No. 5,167,536 to Wang, issued Dec. 1, 1992 and U.S. Pat. No. 5,145,412 to Tan et al., issued Sep. 8, 1992, those patents incorporated herein by reference, show BNC type coaxial connectors having an electrical element connected between the connector shell and either a conductive panel or a printed circuit board for providing a capacitive coupling to the panel or board.
However, while the presence of the capacitor is beneficial in shunting noise to the chassis, the capacitor alone has a limited effect. This is because the capacitor will only work well when it is placed in an environment with relatively high source impedance. If the source impedance is low, the effectiveness of the capacitor is greatly reduced. Thus, there is a need for a device used with a coaxial connector which forces the source impedance to a higher level, to ensure that the capacitor is filtering at an optimal level.
U.S. Pat. No. 4,753,611 to Kobler, issued Jun. 28, 1988, discloses a filtered coaxial assembly including filters for EMI/RFI suppression. These filters have a molded ceramic construction having a lossy ferrite compound dispersed therein. Additionally, that patent describes a method for securing a filter sleeve to the respective inner and outer members of a coaxial cable assembly and assuring mechanical rigidity and electrical contact therebetween. Various embodiments of a filtered connector so produced are described.
U.S. Pat. No. 5,213,522 to Kojima, issued May 25, 1993, discloses a connector with a built-in filter including a ferrite body laving slots corresponding to the connector pins and notches positioned between the slots and a window edge of the case. The notches are contiguous with the slots. Further, chip capacitors are inserted into the notches of the ferrite body, the capacitors being electrically connected between the edge of the shield case and the connector pins.
U.S. Pat. No. 4,995,834 to Hasegawa, issued Feb. 26, 1991, discloses a noise filter connector including an insulation housing, an electrically-conductive shield case, cylindrical capacitors, electrically-conductive post or tab contacts and an inductor block. It is additionally stated that the inductor block is made of a ferrite material and, in connection with the shield case and the capacitors, act as filter devices.
U.S. Pat. No. 4,772,224 to Talend, issued Sep. 20, 1988, discloses a modular electrical connector comprising an insulating body member wherein a plurality of electrical contacts engaged capacitors and which also may be provided with ferrite inductors to produce series inductance.
U.S. Pat. No. 4,952,896 to Dawson, Jr., issued Aug. 28, 1990, discloses a pi-network filter assembly for an electrical connector. The pi-network is comprised of a shunt capacitor at both ends and a series inductor therebetween. Similarly, U.S. Pat. No. 3,597,711 to Buckley, issued Aug. 3, 1971, discloses an electrical connector filter assembly comprising a cylindrical core made of ferromagnetic material and an outer core of dielectric material which is coated by a conductive layer. The assembly provides a removable pi-filter.
U.S. Pat. No. 3,579,155 to Tuchto, issued May 18, 1971, discloses a filtered connector pin contact having a central metal element surrounded by a ferrite ferrule and an outer ceramic sleeve, and including flexible conductive washers end-loading the ferrule and sleeve, to provide flexibility, and prevent breaking.
None of the above references describe a filter for a coaxial connector including a ferrite cylinder concentrically located around the connector shell and a leadless chip capacitor, connected in series, so as to provide improved noise and interference attenuation over a wide range of frequencies. There is a need for such a connector to isolate noise generated in an enclosure from traveling outside the enclosure and along the coaxial-cable to other parts of a computer network.